How do I assign a modifier to all objects using the same material
With the object with the material and modifier as the active object, press Shift+L and select 'Materials'. this will select all objects that share the material. Then press Ctrl+L and select 'Modifiers'. This will give all the objects the same modifiers. 

Note: If you want to make a change to the modifier and have the change across all the objects, you will have to select them all again and hold Alt while making the change.